When providing medical care to patients, it is frequently necessary to monitor the patient using medical diagnostic instruments. One type of instrument, the patient monitor, is capable of monitoring the patient to acquire electrocardiogram data, cardiac output data, respiration data, pulse oximetry data, blood pressure data, temperature data and other parameter data. In particular, lightweight portable monitors exist which can be moved with the patient, allowing continuous monitoring during patient transport.
To facilitate monitoring at remote locations or during patient transport, modern portable patient monitors are powered by rechargeable batteries. Extended-use batteries, with quick recharge times, help maximize monitor availability. Advanced monitors have a smart battery management system which maximizes battery life, reducing maintenance and replacement. Such monitors use smart batteries which can be interrogated to obtain data representing the ongoing state of the battery, e.g., the current charge capacity and battery terminal voltage.
Portable patient monitors with integral battery power supply are commercially available in a compact, ergonomic package which allows easy handling. The compact design is achieved in part through the use of flat display panels. The color or monochrome screen accommodates all numerics and multiple waveforms.
In addition to displaying waveforms and numerics representing the data being acquired, advanced patient monitors are able to display a battery fuel gauge icon representing the current charge capacity of a battery. For example, the battery fuel gauge may display a rectangle of pixels having a width proportional to the percentage of the full charge capacity (assumed to be equal to the design capacity) which remains in the battery. If the full width of the displayed fuel gauge is 75 pixels and the charge remaining is 20% of the full charge capacity, then the current charge state can be depicted by displaying a rectangle on the gauge having a width equal to 15 pixels (i.e., 20% of 75 pixels).
Users of portable battery-powered equipment need to have a dependable means of determining the present condition (state of charge) of the batteries installed in the system. Portable equipment with high-capacity batteries can be capable of very long run times. Some users of the equipment frequently use only a small percentage of this capability, but occasionally can require the equipment to deliver the full run time capability. This type of use profile, along with other contributing factors, such as transient pulsatile loads and electronic circuit errors (circuit tolerances), can cause the electronic charge capacity gauging internal to the smart battery to become inaccurate. This error can result in the smart battery reporting a remaining charge capacity to the host system which is higher than the charge which the battery can actually deliver.
The foregoing problem is most apparent and critical to the user when the actual remaining battery charge capacity is low. With no additional correction means, the charge capacity gauging internal to the smart battery can lead the user to believe that charge remains in the battery when in reality the battery is fully depleted. In this case, the equipment will automatically and unexpectedly shut down. In addition, the charge capacity gauging internal to the smart battery is a very poor indicator of both run time remaining and the rate of decline in the capacity of the battery to power the host system when the battery is nearing a fully discharged condition. The error in the battery charge capacity gauging is relatively small when the battery has a relatively large remaining charge capacity; however, this error becomes unacceptably large as the percentage of the battery charge capacity remaining approaches the percentage of accumulated error in the charge capacity gauging.
Thus there is a need for a method of giving the system user a relative "feel" for the rate of decline in a battery's capability to power a system when the battery is near the fully discharged condition. There is also a need for a method of giving the user a positive point at the end of the fuel gauge where the unit will dependably shut down.